1. Field of Invention
The present invention relates to a modulated light source for optical communication, a wavelength multiplexed optical transceiver, and an optical amplifier. This invention also relates to a wireless communication optical transceiver, and more particularly a wireless communication optical transceiver usable for free space optical transmission. This invention also relates to an active star coupler, and more particularly, an active star coupler regarding free space optical transmission, optical fiber transmission, and transmission via twisted pair cables. Further, this invention relates to an optical communication network including free space optical transmission, optical fiber transmission.
2. Description of Related Art
An optical wireless LAN system has already been commercially available. There is the standard called IrDA (Infrared Data Association) for the point-to-point optical transmission systems using free space light.
Japanese Patent Laid-Open No. Hei. 3-91329 discloses a communication system, which should be called a "cellular network using free space optical signal". FIG. 28 shows the communication system. A mobile station 230 is provided with communication means 231 which adopts a spatial diversity system using free space light. Further, optical stations 232a and 232b, which are base stations for free space optical communication, are provided at important positions. The optical stations 232a and 232b are connected each other by means of a wiring network 237, and are connected to the optical station control station 233. The optical station control station 233 is connected to the public network circuit 236 via the mobile optical telephone exchange station 235.
The mobile station 230 and the control station 233 negotiate via optical stations 232a and 232b, and determine an optical station to be linked. This process is a control process, what is called "hand-over", and is basically the same as the control process of a cellular communication network. In the cellular communication network, adjacent cells use different frequencies to prevent interference between them. The system disclosed in Hei. 3-91329 prevents interference between adjacent cells by means of optical spatial diversity.
Japanese Utility Model Laid-Open No. Hei. 3-92840 discloses a system shown in FIG. 29. This system is provided with plural infrared transmitters 241a through 241c suspended from the ceiling 240. They communicate to each other by means of the horizontal free space light 243, and transmit vertical optical light 244 to the mobile station 242 being below them.
Japanese Patent Laid-Open No. Hei. 3-108823 discloses an optical communication apparatus which combines a transmission optical system having a narrow dispersion angle usable for long-distance communication and a transmission optical system having a wide dispersion angle usable for short-distance communication.
Japanese Patent Laid-Open No. Hei. 3-296332 discloses an interconnectable active star coupler. FIG. 30 shows the star coupler provided with a diode matrix 255 to realize a transmission matrix in which all diagonal elements are set to zero. Signals from input optical fibers 251 are transmitted to output optical fibers 252 via receivers 253, diode matrix 255 and transmitters 254. The diode matrix 255 consists of plural diodes 153, and diodes in the part 154 corresponding to the diagonal elements are removed to realize a transmission matrix in which all diagonal elements are zero. Takeshi Ota, "Coupled star network: A new configuration for optical local area network", IEICE Trans. Commun., Vol. E75-B, No.2, pp.67-75 (1992) also disclose the star coupler in detail.
Japanese Patent Laid-Open No. Hei. 5-3457 (U.S. Pat. No. 5,282,257) disclose an interconnectable passive star coupler. FIG. 31 shows the star coupler provided with an optical waveguide circuit 263 formed in a substrate 262 to realize a transmission matrix in which all diagonal elements are set to zero. In FIG. 31, a reference numeral 261 denotes optical fibers. Takeshi Ota, "Four-port multimode interconnetable star coupler", Electron. Lett., Vol. 29, No. 10, pp.919-920 (1993) also discloses this star coupler in detail.
Hei. 5-3457 further discloses an optical communication network combining interconnectable star couplers and optical fibers.
The above-described optical wireless networks using free space light and point-to-point optical transmission system have been already commercially available. However, the system which links a backbone network to terminals, such as an optical wireless local area network (LAN), has no compatibility with a system communicating between terminals, such as IrDA. Each system is constructed by itself and it is inconvenient for users to deal with each system in a different way.
In recent years development of the plastic optical fiber (POF) of a graded index (GI) type is significant. A GI type POF having the core diameter of 500 m, the numerical aperture (NA) of 0.5 and a transmitting band of 600 MHzkm has been reported (refer to T. Ishigure et al. "High-Bandwidth, High-Numerical Aperture Graded-Index Polymer Optical Fiber" IEEE J. Lightwave Technol, vol. 13, No. 8, pp1686-1691 (1995)). When an optical fiber having a large core diameter and a large numerical aperture is used, it is expected that direct connection of the free space transmitting light with a light transmitting through optical fibers, is facilitated. However, an optical amplifier adaptable to an optical fiber having a large core diameter is difficult to fabricate. In order to provide a constant excited light density in a fiber type optical amplifier, the intensity of an excited light must be increased in proportion to a square of a core diameter, which is unrealistic. In the case of a semiconductor laser amplifier, although the stripe width can be widened comparatively easily, it is difficult in view of the fabrication process to thicken the thickness of the laser amplifier.
A wavelength multiplexed optical transceiver as shown by FIG. 33 has been known (refer to Japanese Published Unexamined Patent Application No. Hei 6-85374). The wavelength multiplexed optical transceiver constituted by an integrated circuit board 11 where optical wave guide paths are integrated, a diffraction grating substrate 14, a photodiode array 15, a semiconductor laser array 16 and an optical fiber 20. A first slab wave guide path (for sending) 12, a second slab wave guide path (for receiving) 13, a first optical coupler 17, a second optical coupler 18 and optical wave guide paths for wirings 19a through 19k are formed on the integrated circuit board 11. Two sets of Fresnel reflecting mirrors 14a and 14b are formed on the diffraction grating substrate 4 and are attached thereto to correspond to the first slab wave guide path 12 and the second slab wave guide path 13, respectively.
The first slab wave guide path 12 and the semiconductor laser array 16 one end face of which is provided with a reflection free coating, are connected by the wave guide paths for wirings 19a through 19e. The wave guide paths for wirings 19a through 19d are connected to a polychromater unit 24 of the first slab wave guide path. The semiconductor laser array 16 is arranged with five semiconductor laser elements. The one end face of the semiconductor laser array 16 is provided with the reflection free coating and the face provided with the reflection free coating is attached to the side of the glass substrate 11. An output unit (common unit) 23 of the first slab wave guide path is connected to the first optical coupler 17. One branch of the optical coupler 17 is connected to the semiconductor laser array 16 via the optical wave guide path for wiring 19e and the other branch is connected to the second optical coupler 18 via the optical guide path for wiring 19e, respectively. A common terminal of the optical coupler 18 is connected to the optical fiber 20.
A laser beam, whose wavelengths are multiplexed can be emitted by the semiconductor laser array 16, the first slab wave guide path 12 and the Fresnel reflecting mirror 14a. The laser beam from the output unit (common unit) 23 of the slab wave guide path 12 is branched by the first optical coupler 17 whereby a portion of the optical output is outputted to the optical fiber 20 via the second optical coupler 18.
The second slab wave guide path 13 and the photodiode array 15 are also connected by the optical wave guide paths for wirings 19h through 19k. Four photodiodes are aligned at the photodiode array 15. An input unit (common unit) 25 of the second slab wave guide path 13 is connected to the second optical coupler 18 by the optical wave guide path for wiring 19g. Further, the optical wave guide paths for wirings 19h through 19k are connected to a polychromater unit 16 of the second slab wave guide path.
An optical signal transmitted from the outside via the optical fiber 20 is branched by the second optical coupler 18 and is led to the second slab wave guide path via the optical wave guide for wiring 19g. The optical signal is divided and focused by the Fresnel reflecting mirror 14b, passes through the optical wave guide paths for wirings 19h through 19k and is converted into electric signals.
It has been reported that a wavelength multiplexed laser oscillator adopted in the sending unit of the wavelength multiplexed optical transceiver cannot conduct high speed modulation since the cavity length is long (refer to M. Zingbl et al.,: "Characterization of a Multiwavelength Waveguide Grating Router Laser", IEEE, Photon. Technol. Lett., vol. 6, No. 9, pp1082-1084 (1994)).
Further, in recent times, a technology referred to as MOPA (Master Oscillator Power Amplifier) has been developed. As schematically shown in FIG. 33, a light from a master oscillator 61 is amplified by a power optical amplifier 64 whereby an output 65 is outputted. A laser beam source having a narrowed band, such as a DFB laser, is frequently used as a master oscillator. The MOPA is often used with a main purpose of promoting the laser output (refer to, for example, Alan Mar et al.,: "Mode-Locked Operation of a Master Oscillator Power Amplifier", IEEE, Photon. Technol. Lett., vol. 6, No. 9, pp1067-1069 (1994)).